1. Field of the Invention
The present invention relates to a process for forming a fine pattern. More specifically, the present invention relates to a process for forming a fine pattern, such as that of an X-ray mask or a wiring of an integrated circuit, which requires a high aspect ratio, by a focused-ion beam (FIB) technology.
2. Description of the Related Art
X-ray lithography, in which an X-ray resist is exposed and developed with an X-ray having a wavelength of several angstroms, is suitable for forming a fine pattern because an X-ray has a short wavelength and does not cause diffraction or interference.
FIG. 1 shows a cross section of an X-ray mask 1, in which a membrane 4 of silicon carbide (SiC) is superimposed on a silicon wafer 3, which in turn, is superimposed on a glass ring 2, and patterns 5 of an X-ray absorbing material of, for example, gold or tungsten (hereinafter referred to as "heavy metal or the like.") are formed on the membrane 4. The X-ray absorbing material patterns 5 are generally formed by electron beam scanning, sputter etching, etc., but recently an ion beam assisted deposition is also partly utilized, in which a film of a heavy metal, or the like, is selectively formed at required location by a focused ion beam technology. In the ion beam assisted deposition, a reaction gas is adsorbed on a surface of a sample, for example, the above-mentioned membrane 4, to which an ion beam is accelerated at an energy of 20 -30 keV and selectively irradiated or scanned to deposit a metal film only at the selectively irradiated portions. For example, tungsten hexacarbonyl W(CO).sub.6 is used as the reaction gas to deposit tungsten. Alternatively, other metals may be deposited by changing the reaction gas. The ion beam assisted deposition deposits films having a fineness depending on a diameter of the ion beam, for example, 0.1 .mu.m or less, and therefore, is suitable for forming or modifying an X-ray mask for which fine patterns are required. Such a fine pattern also may be required for the wiring of integrated circuit devices, and the ion beam assisted deposition is also suitable for forming such wiring, more specifically, for modifying the wiring of an integrated circuit.
In the ion beam assisted deposition, films or patterns are formed by irradiating an ion beam while adsorbing a reaction gas on a sample. The irradiated ion beam causes a deposition of a metal near the desired patterns or the irradiated portion by heat, etc., generated by the irradiation of the ion beam, resulting in a gradual slope along the edge of the patterns. This causes a problem of a low aspect ratio, particularly in an X-ray mask or wiring that requires a fine pattern having a width of 0.1 to 0.5 .mu.m and a thickness of 0.5 to 1.0 .mu.m.
The present invention is intended to remove the above problems and to provide a fine pattern having a high aspect ratio by ion beam assisted deposition.
Japanese Unexamined Patent Publication (Kokai) No. 62-195662, published on Aug. 28, 1987, discloses a process for forming a pattern by ion beam assisted deposition followed by removing an excess portion of the deposited pattern with ion beam etching. This is directed to the same object of the present invention but utilizes a different solution.
In Japanese Unexamined Patent Publication (Kokai) No. 61-123843, published on June 11, 1986, M. Satoh et al disclose an ion beam assisted deposition in which a density of the irradiated ion beam is regulated by controlling an amount or a power of the ion beam irradiation in accordance with an area to be irradiated.